The present invention is drawn to a process and catalyst for the conversion of contaminants in oxygen containing contaminated fluids into less harmful products by irradiating the contaminated fluid with ultraviolet light while the contaminated fluid is in contact with the catalyst.
In spite of decades of effort, a significant need remains for an advanced technology to control stationary source emissions of volatile organic compounds (VOCs) as for example benzene, chlorinated volatile organic compounds (CVOCs) as for example trichloroethlyene, and toxic air pollutants (TAPs) as for example acrylonitrile. A particular need exists for technology which controls emissions from industrial processes and other applications where low concentrations of VOCs and TAPs are present in high flow rate air streams. Similarly, a particular need remains for a technology which can purify aqueous streams containing low concentrations of VOCs.
Dilute stream pollution control is becoming recognized as a major environmental control issue for the United States industrial community at large. For example, the control of air pollution associated with solvent degreasing operations is necessary, including the dilute emissions associated with exhaust ventilation fans. Also, air stripping of contaminated groundwater produces dilute air emissions for which current technology provides no satisfactory solution. Catalytic combustors are available, but require processing tremendous volumes of air and result in uneconomic performance. Thermal incinerators require excessive supplemental fuel for dilute mixtures, and exhibit uncertain selectivity when CVOCs are involved. Gas membrane processes are only now emerging for gas separation, and are ill-suited for dilute mixtures. Pressure swing adsorption using zeolites or resins is not applicable to dilute mixtures, and rotating wheel adsorbers are uneconomic for such dilute concentrations of organics. Packed bed activated carbon adsorption is widely practiced, but creates a hazardous solid waste which is increasingly difficult to manage. Carbon regeneration by steam is costly, and is generally economic only for very large scale operations. Landfill options for spent carbon will become more limited, as it involves transportation and disposal of hazardous wastes, particularly for CVOC applications. Similar deficiencies are exhibited in aqueous phase treatment systems.
Control of indoor air pollution is also of growing importance, with the objective of enhancing workplace environmental health and safety protection. The Occupational Safety and Health Administration (OSHA) is promulgating new regulations to reduce workplace exposure to indoor air contaminants such as CVOCs. Many CVOCs are particularly toxic. Certain CVOCs are suspected carcinogens, others are linked to possible birth defects, and still others are suspected active precursors in the destruction of the stratospheric ozone layer. Of the 189 targeted air toxics in the Clean Air Act Amendments of 1990, about one-third of the compounds are chlorinated. By the standards of conventional air pollution control technology, indoor air pollution is at exceedingly dilute concentrations.
The industrial demand for ultrapure water is also growing rapidly, requiring development of a technology which effectively removes organic contaminates at very low concentrations. For example, integrated circuits require highly purified water for rinsing semiconductor wafers, controlled to a total organic carbon level of 50 ppb. Reduction of the organic content of water to these levels is not readily achievable with existing technology.
In spite of considerable efforts of researchers in the field, most UV photocatalysts exhibit shortcomings in catalyst activity, selectivity, and deactivation which limit their commercial utility for air and water pollution control.
Researchers have studied titania photocatalysts which have been used for decades (See Formenti, M., et al., "Heterogeneous Photocatalysis for Oxidation of Paraffins", Chemical Technology 1, 680-686, 1971, and U.S. Pat. No. 3,781,194 issued Dec. 25, 1973). Raupp et al have reported titania photocatalysts for the UV oxidation of organics in air. The activity of a titania photocatalyst rapidly declines with time-on-stream with trichloroethylene (TCE) in air (See Raupp, G. B., et al., "Destruction of Organics in Gaseous Streams Over UV-Excited Titania", 85.sup.th Annual Meeting, Air & Waste Management Association, Kansas City, Jun. 21-26, 1992; and Raupp, G. B., "Photocatalytic Oxidation for Point-of-Use VOC Abatement in the microelectronics Fabrication Industry", Air & Waste Management Association, 87.sup.th Annual Meeting, Cincinnati, Ohio, Jun. 19-24, 1995). Ollis et al. Describes photocatalytic reactors and reports that TCE photocatalysis in air can lead to 75 ppm(v) of phosgene in the reactor product in the photocatalytic oxidation of organics over a thin titania bed (See Ollis, D. F., in Photocatalytic Purification and Treatment of Water and Air, 481-494, Elsevier N.Y., 1993; and Peral, J. and D. F. Ollis, "Heterogeneous Photocatalytic Oxidation of Gas-Phase Organics for Air Purification", J. Catalysis, 136, 554-564, 1992). Researchers at Purdue have investigated gas phase photocatalysis of TCE using titania on a concentric reactor wall around a UV light source. They employed residence times of over 6 seconds, and found evidence of byproducts, suspected to be phosgene (See Wang, K. and B. J. Marinas, in Photocatalytic Purification and Treat. of Water and Air, 733-737, Elsevier, 1993). Teichner et al. report that byproduct formation with titania photocatalysts is the rule, not the exception (See Teichner, S. J. and N. Formenti, in Photoelectrochemistry, Photocatalysis and Photoreactors, 457-489, Reidel Pub, Boston, 1985). Nutech Energy Systems has disclosed titania impregnated on a fiberglass mesh (See U.S. Pat. No. 4,892,712 issued Jan. 9, 1990; U.S. Pat. No. 4,966,759 issued Oct. 30, 1990; and U.S. Pat. No. 5,032,241 issued Jul. 16, 1991). A technical paper on the Nutech technology disclosed a gas phase residence time of 8.4 seconds and evidence of byproducts formation up to 34 seconds residence time (See Al-Ekabi, H., et al., in Photocatalytic Purification & Treatment of Water and Air, 719-725, Elsevier, N.Y., 1993). The University of Wisconsin investigators have used 100 second residence time for photocatalytic destruction of gas phase TCE using titania (See Yamazaki-Nishida, S., et al., "Gas Phase Photocatalytic Degradation on Titania Pellets of Volatile Chlorinated Organic Compounds from a Soil Vapor Extraction Well", J.Soil Contamination, September, 1994; Fu, X., W. A. Zeltner, and M. A. Anderson, "The Gas-Phase Photocatalytic Mineralization of Benzene on Porous Titania-Based Catalysts", Applied Catalysis B: Environmental, 6, 209-224, 1995; U.S. Pat. No. 5,035,078 issued Jul. 30, 1991).
Due to the high absorption of UV light by titania about 99% of the incident UV radiation is absorbed within the first 4.5 microns on titania (See Peral, J. and D. F. Ollis, "Heterogeneous Photocatalytic Oxidation of Gas-Phase Organics for Air Purification", J. Catalysis, 136, 554-564, 1992). Hence, the patent literature discloses process to distribute the titania in thin layers in an attempt to overcome this deficiency. In 1973, Teichner disclosed a process of using titania supported on a matrix in a thin film reactor to oxidize hydrocarbons to aldehydes and ketones (See U.S. Pat. No. 3,781,194 issued Dec. 25, 1973). Titania has been deposited in thin layers on glass wool (See U.S. Pat. No. 4,888,101 issued Dec. 19, 1989), on a ceramic membrane (See U.S. Pat. No. 5,035,078 issued Jul. 30, 1991), and the wall of a reactor (See U.S. Pat. No. 4,966,665 issued Oct. 30, 1990). Raupp et al. Has disclosed titania for photocatalytic use when mixing two gas streams, and included thin bed catalytic reactors (See U.S. Pat. No. 5,045,288 issued Sep. 3, 1991).
U.S. Pat. No. 4,966,665 discloses the use of titanium dioxide as a photocatalyst for destruction of chlorine-containing organic compounds in an oxygen-bearing vent gas. The process was used in a system wherein the titanium dioxide was supported on the wall of a reactor through which the vent gas is passed. This is an ineffective reactor design for large gas flows, because each UV bulb must be located in its own concentric tube coated with catalyst, requiring a very large number of small diameter parallel tubes among which the flow must be equally balanced. Further, the cost of such a reactor is very high compared to a single large diameter reactor. Still further, when a vent gas containing 30 ppm TCE was contacted with titanium dioxide for 26 seconds, 90% destruction of TCE was obtained. However, the analysis of the reactor products revealed the formation of byproducts such as 4 ppm phosgene and 1 ppm carbon tetrachloride which are more hazardous than TCE.
U.S. Pat. No. 5,045,288 discloses a process of removing organic contaminants from a gaseous stream, wherein a mixing step is conducted, combining (a) a gaseous oxygen bearing stream and (b) a contaminated stream. This mixture is then passed over a photocatalyst exposed to UV radiation of wavelength not greater than 600 nm. The reaction conditions must be preselected to prevent formation of a liquid phase on the catalyst. Only when the reactor comprises a body portion having a window for passing visible light, the photocatalyst may be selected from the group consisting of titanium dioxide, zirconium oxide, antimony oxide, zinc oxide, stannic oxide, cerium oxide, tungsten oxide, and ferric oxide. The invention is deficient in that it requires mixing two streams, requires avoiding liquid phases, and requires a visible light window, all of which are impractical in commercial systems. Further, the catalyst compositions are not effective.
U.S. Pat. No. 4,888,101 discloses a system for photocatalysis wherein a semiconductor is entrapped in a fiber mesh. U.S. Pat. Nos. 4,892,712 and 4,966,759 disclose a photocatalyst for detoxifying organic pollutants from a fluid, comprising a substrate in the form of a plurality of layers of a filamentous, fibrous, or stranded base material, and a photoreactive metal semiconductor material bonded to surfaces of said layers. The photoreactive material is selected from anatase, CdS, CdSe, ZnO.sub.2, WO.sub.3, and SnO.sub.2. These disclosures are of limited value because the substrate leads to a high pressure drop under conditions providing intimate contacting of the fluid flowing through or by the substrate. Further, the base material is generally of low surface area, leading to poor overall reactor performance. Still further, the design also generally requires a sleeve of this material around each bulb, leading to a costly reactor design. Finally, the photoreactive metal semiconductors are not of high photocatalytic activity. U.S. Pat. No. 5,032,241 provides a similar system for killing microorganisms in a fluid.
U.S. Pat. No. 4,780,287 discloses a process of decomposing volatile organic halogenated compounds by passing the gas through silica gel or quartz chips, and the bed is thereafter irradiated with UV light. Ozone or hydrogen peroxide are added to an aqueous phase. The system lost catalytic activity and required heating with nitrogen gas to restore the decomposing activity. Presumably, this loss of activity was due to the decomposition of the CVOC, rather than promoting its oxidation as disclosed in the present application. U.S. Pat. No. 4,941,957 discloses a process for decomposing CVOCs present in gases and aqueous solutions. The aqueous phase CVOCs are removed by volatilizing into a gaseous carrier. The gas was then passed through silica gel and simultaneously irradiated with UV light and/or exposing the bed to ozone. The system required a great excess of ozone, using 2% ozone to decompose TCE at air compositions of only 50 parts per billion (ppb) to 700 ppb, which is generally uneconomic.
U.S. Pat. No. 5,468,699 discloses a process of forming a semiconductor material such as titanium dioxide incorporating molecular sieve material. A 10 ppm aqueous solution of TCE was mineralized to chloride ions in a flow reactor. An excessively long residence time of 1 hour was required for mineralization.
U.S. Pat. No. 4,863,608 discloses a photocatalyst comprising an inorganic semiconductor selected from TiO.sub.2, SrTiO.sub.3, and CdS in fine particulate form, and a noble metal or an oxide thereof selected from the group consisting of Pt, Pd, Ru, RuO.sub.2 and Rh deposited on said semiconductor particle. A process is disclosed for purifying water using said photocatalyst, particularly the last trace amount of organic carbon impurities. However, this technology requires excessive reaction time for water purification, of up to 60 minutes.
Accordingly, it is a principle object of the present invention to provide a process and an improved photocatalyst for efficient oxidative destruction of contaminants in oxygen containing contaminated fluid streams utilizing ultraviolet light.
It is a further object of the present invention to provide a process and catalyst as mentioned above which is relatively easy and inexpensive to implement and carry out.